The present invention relates to an improved detection device for use in a magnetic bearing of the radially active/ axially passive type for detecting both radial and tilting displacements of the rotor member of the bearing in relation to the stator member thereof.
A magnetic bearing apparatus which this invention is concerned with comprises stator and rotor members coaxially disposed one around the other without mechanical contact therebetween. The stator or the rotor member bears a permanent magnet, the magnetic flux of which ensures the axial alignment of the rotor member relative to the stator member. The latter member also bears electric coils positioned such that two of these coils are centered on each axis in the common plane of the stator and rotor members. These coils generate in the annular radial gap between the stator and the rotor members, magnetic fields serving in a manner known per se to control the radial position of the rotor member relative to the stator member along the two orthogonal axes in their common plane. For that purpose, the pair of coils controlling the radial position along a same axis are usually connected in series or in parallel in an electronic control circuit. When the rotor member is radially displaced along e.g. the X-axis relative to its centered position with respect to the stator member, the position deviation signal is electronically processed such that an electric current flows through the two coils controlling the X-axis in one sense or the opposite depending on the polarity of the position deviation signal. The resulting magnetic fields then are such that the magnetic flux of the permanent magnet is increased in the wider gap and is decreased in the smaller gap. As a consequence, the rotor member is attracted more strongly towards the stator member where the gap is wider, thereby restoring the rotor member to its centered position.
A magnetic suspension apparatus of this type necessarily comprises a detection device for detecting the radial position of the rotor member in relation to the stator member and for generating the electric position deviation signals to be used to correct the position of the rotor member.
The known position detection devices are devices of comparatively small dimensions relative to the dimensions of the rotor member used in the magnetic bearings considered for space applications. Generally, use is made of position detection devices of the electro-optic type as described for instance in the U.S. Pat. Nos. 4,000,929 and 4,043,614. However these prior art detection devices used for this purpose generally suffer from one or more of the following important limitations.
Most of the detection devices employ position sensitive elements which have relatively small physical dimensions in relation to the size of the rotor member. This causes these devices to exhibit a relatively high level of noise due to the rotor surface irregularities and the rotor shape error. This noise is particularly troublesome when differentiating networks are used in the magnetic bearing suspension control electronics.
In general, the prior art position detection devices are designed so as to be insensitive to all but radial displacements of the rotor member. When these devices are incorporated in magnetic bearings this usually means that they provide very little useful signals for controlling the tilting motions and, therefore, they do not make good sensors for use in systems for actively damping such tilting motions.
Most of the prior art position detection devices have relatively complex electronic circuits associated therewith. This is especially true for alternating current devices where phase sensitive demodulation circuits are normally employed. This complexity has a negative influence on the reliability of the suspension bearing and the ease with which it can be brought to an operational state.
The above limitations are overcome with the radial position detection device according to the invention.